1. Field of the Invention
The present invention relates to an amplifier used in radio frequency electronics, and more specifically, to a low noise amplifier in radio frequency electronics.
2. Description of the Related Art
As development of wireless communication technology has been accelerating, the rapid progressions in the wireless communication devices succeed in reaching the ubiquitous connection possible. Radio Frequency (RF) transceivers are important components for wireless communication devices. The majority of the RF Integrated Circuits (ICs) used in the wireless communication devices were implemented using either GaAs or silicon bipolar technologies. In recent years, when the Complementary Metal-Oxide-Semiconductor (CMOS) technology brought the cutoff frequency (fT) of MOS transistors up to multi-tens of GHz, such RF ICs are capable of being built in CMOS technology. An advantage of using CMOS radio frequency transceivers is to provide digital functions easily. As a result, it is possible to incorporate the whole system on one single chip yielding to low cost wireless devices. Low Noise Amplifiers (LNAs) are one of the most important building blocks in the front end of wireless communication devices, and are used in a wide variety of applications such as wireless RF communication devices including wireless computer networks and mobile phones. For LNAs, the gain linearity applied to a signal is an important operating characteristic, especially when the input signal becomes higher. Basically, the low noise amplifier must meet simultaneous low noise and high linearity, and the noise contribution from the LNA may be negligible compared to the input signal. In fact, the linearity of the LNA becomes the most important factor. The gain linearity is typically related to the transconductance of a MOSFET in an input stage of the amplifier. For example, the transconductance of a MOSFET operating in the saturation region is constant only when the input signal is lower; when the input signal is higher, the transconductance may vary as a function of the input signal, thus, leading to nonlinear amplification of the signal. In order to overcome the problem of nonlinear amplification, source degeneration may be employed to increase the linearity of the input stage. However, at higher frequencies source degeneration may not be effective due to the large parasitic capacitance of the device. Also, source degeneration may increase power consumption due to the relative low gm/Id for the MOSFET in comparison with a bipolar device. Traditionally, the use of an inductor is a way to achieve the purpose of source degeneration. Nevertheless, because the inductor has larger area, and the impedance of the inductor is associated with its operating frequency. In addition, gain control is also very important in practical applications since the gain of the LNA could vary with process and temperature if not properly controlled.